The present invention relates to an uninterruptible power supply unit capable of continuously supplying electric power to a load using energy stored in electric power storage means when power service is interrupted.
As an uninterruptible power supply unit of this type, for example, an uninterruptible power supply unit based on a so-called SPS (stand-by power supply) system with high operating energy efficiency has been proposed. FIG. 8 shows an example thereof in which AC power from a commercial power supply source is supplied to a load (not shown) through a relay switch 71. A half-bridge-type converter 72 and a DC power supply source 73 are-connected to a power supply line for supplying AC input to the load in parallel relative to an AC power input terminal of the AC power from the commercial power supply source. Further, a reactor 74 is disposed between the power supply line and the converter 72. Two switching elements connected in series and two capacitors connected in series, for example, are connected to the converter 72 in parallel. Also, each of the switching elements is connected to a diode in opposite parallel.
When the AC power is supplied normally, the relay switch 71 is controlled in an electrically conductive state to supply the AC input to the load. On the other hand, when an abnormal condition occurs in the AC input due to power outage and the like, the relay switch 71 is controlled to shut off to disconnect a side of the commercial power supply from the power supply line. Then, using the DC power supply source 73 as an energy source, the converter 72 is operated as an inverter to generate specific AC power and supply the AC power to the load.
With the configuration described above, even in the event of power outage, AC power can be continuously supplied to the load.
Incidentally, in the SPS-type uninterruptible power supply unit described above, when the SPS system responds and shuts off the relay switch 71 in the event of power outage, there is a certain time lag about a few milliseconds to more than 10 milliseconds from a time when the power outage is detected to a time when the relay switch 71 is shut off.
At this time, there may be a case in which the uninterruptible power supply unit itself or a device linked to the same system may be adversely affected by over current flowing through a short-circuit portion.
Alternatively, in the event of power outage, a method has been proposed in which the converter 72 is temporally stopped to shut off the relay switch 71, and then the converter 72 is activated again. However, in this method, since the converter 72 is stopped until the relay switch 71 is shut off, there is a problem called “temporal shut off” in which the voltage output to the load is temporally stopped.
To prevent “temporal shut off”, it is necessary to continuously operate the converter 72. In this case, however, it is possible to adversely affect a device due to the over current described above.
Accordingly, it is desired to provide a device and a method in which the above two problems are simultaneously solved.
Accordingly, the present invention has been created to solve the above problems, and an object of the present invention is to provide an uninterruptible power supply unit capable of preventing the adverse effect on a device due to the over current and the temporal shut-off of the output voltage when the AC power input side is disconnected upon a decline in the input voltage.
Further objects and advantages of the invention will be apparent from the following description of the invention.